Magnetometers (i.e. magnetic field detectors) are utilized in a number of applications including applications for detecting the earth's magnetic field, and in particular the direction thereof. One such application is in long multi-element towed acoustic arrays utilized for submarine detection or other types of marine detection where the orientation of the acoustic detectors must be accurately established in order for optimum sensitivity to be achieved. A plurality of magnetometers spaced along the array are utilized to perform this function.
However, as the signal processing equipment utilized with magnetometers becomes more sophisticated, even relatively small errors in magnetometer readings become significant. For example, in a towed array application, accuracy in the detection of the direction of the earth's magnetic field to one tenth of a degree, or even one hundredth of a degree, may be desirable.
However, in an application such as the towed array application previously described, it is necessary to provide a significant amount of DC current to various components through wires which pass relatively close to the magnetometers. The magnetic field caused by this current flow in the area of the magnetometer causes unacceptable errors in the readings therefrom and thus in the attitude detection for the array.
A need therefore exists for a relatively simple and inexpensive method and apparatus to permit a magnetometer to be utilized in the vicinity of DC current carrying conductors without the magnetic field of such conductors adversely affecting the accuracy of the magnetometer output.
One prior proposal for achieving this objective was to utilize coaxial conductors, the central conductor carrying current in one direction and the concentrically positioned outer conductor carrying an equal and opposite current. Such an arrangement does not result in the creation of an external magnetic field. However, the coaxial conductor approach does not provide satisfactory results in applications such as for a towed array for a number of reasons. First, the space in such arrays is limited. Since the center conductor of the coaxial cable cannot be made arbitrarily small without unacceptable power losses due to resistance of the cable, a coaxial conductor is not an ideal solution to the magnetic interference problem in many applications. A more serious problem is that perfect coaxial cables are a mathematical concept, and while elimination of external magnetic fields can be approached using coaxial cables, the elimination of these fields using coaxial cables cannot generally be achieved. This is particularly true in a towed array situation where the array is flexing in use and is also stowed on a drum. This requires considerable flexibility of the coaxial cable and generally requires that the outer shield of the coaxial cable be braided from fine wires touching each other. This causes current distribution through the shield to not be perfectly uniform, resulting in less than total cancellation of external magnetic fields. Therefore, such cables will show small residues of magnetic field from place to place along the cable and from time to time.